The present invention relates generally to manufacturing methods for control devices of the type in which a controlled parameter set point is determined by the position of a potentiometer wiper and in which the set point is indicated by a movable element associated with the wiper positioning member in cooperation with a fixed element on the device, and more particularly to a method for manufacturing such devices incorporating wide tolerance potentiometers and fixed indicator elements having predetermined orientations which are uniform from device to device.
A wide variety of control devices are known in which a parameter is controlled to a manually entered set point. Depending on the type of control device, the set point may be entered through any of several kinds of mechanisms. In electrical systems, common input mechanisms include potentiometers, switch keypads and digital encoders.
Particularly in high volume consumer devices, cost of the input mechanism is an important consideration. Using presently available technology, ceramic metal (cermet) potentiometers can be produced quite inexpensively. Particularly when the auxiliary elements which may be required with keypad or digital encoder mechanisms are considered, low end cermet potentiometers provide for the lowest cost input mechanisms in many applications.
A complicating factor in many applications is that the lower cost cermet potentiometers are typically wide tolerance devices because, with the materials and assembly techniques used, it is difficult to manufacture such potentiometers without significant part-to-part variations. A single model of a typical low cost cermet potentiometer may exhibit full scale resistance variations of greater than 20% and linearity variations of greater than 10%. Depending on the application, input mechanisms exhibiting such variations may be undesirable or totally unacceptable.
Potentiometers having much tighter tolerance specifications are available. However, potentiometers exhibiting part-to-part consistency within a 1% tolerance typically cost in excess of four times the cost of inexpensive potentiometers. At that cost, a membrane switch keypad or digital encoder with any necessary auxiliary elements becomes relatively more cost effective.
Although a potentiometer model of inexpensive cermet construction may exhibit substantial part-to-part variations, it has been found that each individual potentiometer is monotonic and linear over its range except for the regions of the end point contact metalization pads. Thus, complications introduced by nonlinearities can be avoided through the use of designs which exclude usage of the nonlinear end regions. Also, full scale resistance variations can be ignored or easily accommodated if such potentiometers are used as voltage dividers rather than variable resistors.
A further advantage of a potentiometer over a switch keypad in certain applications is that the keypad, per se, does not provide an indication of the commanded set point. If the commanded set point is to be displayed, an auxiliary display unit must be provided. Also, a switch keypad in conjunction with a microprocessor is inherently at least somewhat volatile, and could lose the commanded set point if electrical power is lost.
In accordance with the foregoing discussion, there are many applications in which an inexpensive cermet potentiometer would form a quite acceptable input mechanism if part-to-part variations, nonlinearities in resistance characteristics at the end point contact regions and full scale resistance variations could be avoided or accommodated. The applicants have devised a device design and method of manufacture which satisfactorily addresses these issues to permit the use of inexpensive, wide tolerance potentiometers in a manner which will provide satisfactory performance and acceptable human factors function and appearance for the operator interface.